Rotation speed control for recording information

ABSTRACT

A disc recording device has a mode control unit ( 31 ) for switching the device either to a read mode or to a write mode. Reading and writing can be alternated for simultaneous recording and reproducing of video. A rotation speed control unit ( 37 ) sets the rotation speed of the record carrier, and has a speed selector ( 38 ) for selecting one of at least two speed settings for the read mode in dependence on an actual rotation speed of the record carrier during the write mode when switching from write mode to read mode. By setting the read speed to one of a limited number of settings the difference in rotation speed between said actual rotation speed and the speed in the read mode is limited. Due to the limited differences power dissipation is limited, while the performance of access time and playability of real-life discs is at a high level.

The invention relates to a device for recording information on adisc-shaped record carrier, the record carrier comprising a track forrecording information, the device comprising a head for scanning thetrack, a read unit for retrieving information from the track via thehead, a write unit for recording information in the track via the head,a mode control unit for switching the device either to a read mode or toa write mode, and a rotation speed control unit for setting the rotationspeed of the record carrier.

The invention further relates to method of controlling a speed ofrotation of a disc-shaped record carrier, the record carrier comprisinga track for recording information, the method comprising scanning thetrack via a head, retrieving information from the track via the head,recording information in the track via the head, switching the deviceeither to a read mode or to a write mode, and setting the rotation speedof the record carrier.

A device for recoding information is known from patent applicationJP11-306662. The device has a head and a write unit for recordinginformation in a track on a record carrier like an optical disc and aread unit for reading information. The device also has a mode controlunit for switching the device to a read mode or to a write mode, and aspeed control unit for controlling the rotational speed of the recordcarrier. During the write mode the rotational speed of the recordcarrier is controlled according to a constant linear velocity (CLV)profile, i.e. the linear speed of the track versus the head is constantand hence the rotational speed of the disc varies in inverse proportionto the radial position of the head.

Such a CLV speed profile is common for optical discs like CD-R, DVD+RWor DVD-RW.

After a period of writing the mode is switched to the read mode. Theinformation to be read usually is located at a different radialposition, and applying the CLV profile also for the read mode wouldrequire a change of rotation speed according to the new radial positionof the head. In the device the speed control unit sets the rotationalspeed at a fixed value equal to the actual speed during write mode justbefore switching. Hence the rotational speed in read mode is the same asthe actual speed in the write mode. The rotational speed in read mode isnot changed, and therefore the data rate of the information from thetrack will vary with the radial position. Such speed control is usuallycalled constant angular velocity (CAV). The effect of keeping the samerotation speed when switching from write mode to read mode is that nodelay occurs in accessing the data due to speed changes. However the CAVspeed profile is not very suitable for use in a high performancerendering device. In particular the read process may be unreliablebecause of the large range linear speeds occurring due to keeping thesame speed when switching from write mode.

Therefore it is an object of the invention to provide a device andmethod for controlling the rotation speed that is more reliable andsuitable for use in a high performance rendering device.

According to a first aspect of the invention the object is achieved witha device as defined in the opening paragraph, characterized in that therotation speed control unit comprises a speed selector for selecting oneof at least two speed settings for the read mode in dependence on anactual rotation speed of the record carrier during the write mode whenswitching from write mode to read mode, the difference in rotation speedbetween said actual rotation speed and the speed in the read mode beinglimited by said selection. According to a second aspect of the inventionthe object is achieved with a method as defined in the openingparagraph, characterized in that the method comprises selecting one ofat least two speed settings for the read mode in dependence on an actualrotation speed of the record carrier during the write mode whenswitching from write mode to read mode, the difference in rotation speedbetween said actual rotation speed and the speed in the read mode beinglimited by said selection. The effect of providing a limited number ofsettings for the rotational speed is that the device can be designed andtested for optimal performance at the limited range of speeds whenreading.

The invention is also based on the following recognition. The knownsystem provides a solution for avoiding delays in access time whenswitching modes. However the range of speeds during reading can be aslarge as the difference between the maximum and minimum rotational speedduring recording, e.g. a range of about 2.5 times in the CD or DVDsystem. The inventors have seen that such a large range of speedsrequires a complex system design for overall performance at all speedsof the data recording and reading process, in particular duringsimultaneous recording and retrieving data streams via buffers byalternatingly switching modes. By using a limited number of settings forthe rotational speed which are located between the minimum and maximumspeed during recording, the complexity of system design is reducedsignificantly. In particular the testing of the total disc scanningsystem in the presence of several error sources, usually calledplayability testing, can be reduced because of the limited number ofspeeds. Further it has been noted the energy required by the drivingmotor to change the rotational speed of the disc increases with thesquare said change. But by providing said limited number of settings thedifference in the rotational speed is limited. Hence by limiting thespeed differences the energy dissipated in the device for changingspeeds is only slightly higher than in the prior art device describedabove. Further the control unit is less complicated because only alimited number of settings are needed for parameters that depend on thereadout speed, for example the gain in the motor control loop.

In an embodiment of the device the speed control unit is arranged forcontrolling the speed of the record carrier during recording accordingto a constant linear velocity (CLV) profile. This has the advantage thatthe linear speed of the head versus the track is constant during therecording process. The recording process usually is more criticalregarding design parameters and tolerances against adverse circumstanceslike dust. Due to the constant speed the recording process can beoptimized for this speed.

In an embodiment of the device the speed control unit is arranged forcontrolling the speed of the record carrier during reading according toa constant angular velocity (CAV) profile. This has the advantage thatthe rotation speed during reading can be easily set at a fixed valuewhen switching to reading mode.

In an embodiment of the device the speed selector comprises a lowestspeed setting for a rotation speed substantially above the lowestrotation speed in the write mode. This has the advantage that therotation speed during reading never is below the speed of the lowestsetting and above the lowest rotation rate during recording. Hence theminimum reading speed which has to be included in the system and bufferdesign is higher than the lowest recording speed.

In an embodiment of the device the speed selector comprises a highestspeed setting for a rotation speed substantially below the highestrotation speed in the write mode. This has the advantage that therotation speed during reading is within a range which is smaller thanthe range of rotation speeds during recording. Reading at such rotationspeeds may occur at any radial position. Hence the linear readingvelocity of the head versus the track during reading is limited.

Further preferred embodiments of the device and server entity accordingto the invention are given in the further claims.

These and other aspects of the invention will be apparent from andelucidated further with reference to the embodiments described by way ofexample in the following description and with reference to theaccompanying drawings, in which

FIG. 1 a shows a record carrier (top view),

FIG. 1 b shows a record carrier (cross section),

FIG. 2 shows a recording device,

FIG. 3 shows speed setting points,

FIG. 4 shows simultaneous recording and retrieving by alternatinglywriting and reading, and

FIG. 5 shows rotations speeds during alternatingly writing and reading.

In the Figures, elements which correspond to elements already describedhave the same reference numerals.

FIG. 1 a shows a disc-shaped record carrier 11 having a track 9 and acentral hole 10. The track 9, being the position of the series of (tobe) recorded marks representing information, is arranged in accordancewith a spiral pattern of turns constituting substantially paralleltracks on an information layer. The record carrier may be opticallyreadable, called an optical disc, and has an information layer of arecordable type. Examples of a recordable disc are the CD-R and CD-RW,and writable versions of DVD, such as DVD+RW, and the high densitywritable optical disc using blue lasers, called Blue-ray Disc (BD).Further details about the DVD disc can be found in reference: ECMA-267:120 mm DVD—Read-Only Disc —(1997). The information is represented on theinformation layer by recording optically detectable marks along thetrack, e.g. crystalline or amorphous marks in phase change material. Thetrack 9 on the recordable type of record carrier is indicated by apre-embossed track structure provided during manufacture of the blankrecord carrier. The track structure is constituted, for example, by apregroove 14 which enables a read/write head to follow the track duringscanning. The track structure comprises position information, e.g.addresses, for indication the location of units of information, usuallycalled information blocks. The position information includes specificsynchronizing marks for locating the start of such information blocks.The position information is encoded in frames of modulated wobbles asdescribed below.

FIG. 1 b is a cross-section taken along the line b-b of the recordcarrier 11 of the recordable type, in which a transparent substrate 15is provided with a recording layer 16 and a protective layer 17. Theprotective layer 17 may comprise a further substrate layer, for exampleas in DVD where the recording layer is at a 0.6 mm substrate and afurther substrate of 0.6 mm is bonded to the back side thereof. Thepregroove 14 may be implemented as an indentation or an elevation of thesubstrate 15 material, or as a material property deviating from itssurroundings.

The record carrier 11 may be intended for carrying informationrepresenting digitally encoded video according to a standardized formatlike MPEG2.

FIG. 2 shows a recording device for writing information on a recordcarrier 11 of a type which is writable or re-writable, for example CD-Ror CD-RW, or DVD+RW or BD. The device is provided with recording meansfor scanning the track on the record carrier which means include a driveunit 21 for rotating the record carrier 11, a head 22, a positioningunit 25 for coarsely positioning the head 22 in the radial direction onthe track, and a control unit 20. The head 22 comprises an opticalsystem of a known type for generating a radiation beam 24 guided throughoptical elements focused to a radiation spot 23 on a track of theinformation layer of the record carrier. The radiation beam 24 isgenerated by a radiation source, e.g. a laser diode. The head furthercomprises (not shown) a focusing actuator for moving the focus of theradiation beam 24 along the optical axis of said beam and a trackingactuator for fine positioning of the spot 23 in a radial direction onthe center of the track. The tracking actuator may comprise coils forradially moving an optical element or may alternatively be arranged forchanging the angle of a reflecting element. For writing information theradiation is controlled to create optically detectable marks in therecording layer. For reading the radiation reflected by the informationlayer is detected by a detector of a usual type, e.g. a four-quadrantdiode, in the head 22 for generating a read signal and further detectorsignals including a tracking error and a focusing error signal forcontrolling said tracking and focusing actuators. The read signal isprocessed by read processing unit 30 of a usual type including ademodulator, deformatter and output unit to retrieve the information.Hence retrieving means for reading information include the drive unit21, the head 22, the positioning unit 25 and the read processing unit30. The device comprises write processing means for processing the inputinformation to generate a write signal to drive the head 22, which meanscomprise an input unit 27, and modulator means comprising a formatter 28and a modulator 29. The control unit 20 controls the recording andretrieving of information and may be arranged for receiving commandsfrom a user or from a host computer. The control unit 20 is connectedvia control lines 26, e.g. a system bus, to said input unit 27,formatter 28 and modulator 29, to the read processing unit 30, and tothe drive unit 21, and the positioning unit 25. The control u nit 20comprises control circuitry, for example a microprocessor, a programmemory and control gates, for performing the procedures and functionsaccording to the invention as described below. The control unit 20 mayalso be implemented as a state machine in logic circuits. The input unit27 receives the user real-time information and processes the audioand/or video to blocks of information, which are passed to the formatter28 for adding control data and formatting the data according to therecording format (as described below), e.g. by adding error correctioncodes (ECC) and/or interleaving. For computer applications units ofinformation may be interfaced to the formatter 28 directly. Theformatted data from the output of the formatter 28 is passed to themodulation unit 29, which comprises for example a channel coder, forgenerating a modulated signal which drives the head 22. Further themodulation unit 29 comprises synchronizing means for includingsynchronizing patterns in the modulated signal. The formatted unitspresented to the input of the modulation unit 29 comprise addressinformation and are written to corresponding addressable locations onthe record carrier under the control of control unit 20. The controlunit 20 is arranged for recording and retrieving position dataindicative of the position of the recorded information volumes. Duringthe writing operation, marks representing the information are formed onthe record carrier. The marks may be in any optically readable form,e.g. in the form of areas with a reflection coefficient different fromtheir surroundings, obtained when recording in materials such as dye,alloy or phase change material, or in the form of areas with a directionof magnetization different from their surroundings, obtained whenrecording in magneto-optical material. Writing and reading ofinformation for recording on optical disks and usable formatting, errorcorrecting and channel coding rules are well-known in the art, e.g. fromthe CD system. The marks can be formed by means of the spot 23 generatedon the recording layer via the beam 24 of electromagnetic radiation,usually from a laser diode. In an embodiment the input unit 27 comprisescompression means for input signals such as analog audio and/or video,or digital uncompressed audio/video. Suitable compression means aredescribed for video in the MPEG standards, MPEG-1 is defined in ISO/IEC11172 and MPEG-2 is defined in ISO/IEC 13818. The input signal mayalternatively be already encoded according to such standards.

The device has a mode switching unit 31 coupled to the control unit 20via the system bus 26. The mode switching unit switches the deviceeither to a read mode or to a write mode, in dependence of user commandsor automatically for accommodating a number a data streams. The controlunit 20 has a rotation speed control unit 37 for setting the rotationspeed of the record carrier. In an embodiment the rotation speed duringrecording is controlled according to a constant linear velocity (CLV)profile, in which profile the speed of the track versus the head ismaintained at a constant linear speed. The rotation speed increases ininverse proportion to the radial position as shown in FIG. 3. In anembodiment the rotation speed during reading is controlled according toa constant angular velocity (CAV) profile, in which profile the rotationspeed is kept at a constant value, and the data speed increases inproportion with the radial position of the head 22.

The rotation speed control unit 37 includes a speed selector 38 forselecting one of at least two speed settings for the read mode independence on an actual rotation speed of the record carrier during thewrite mode. The actual speed is determined when switching from writemode to read mode under control of the mode switch unit 31. The selectoris controlled to select a rotation speed such that the difference inrotation speed between said actual rotation speed and the speed in theread mode is limited by said selection. It is noted that spinning up ofthe record carrier requires motor power more than proportionallyincreasing with increasing difference in speed. By using the limitednumber of setting points the difference in speed is limited. Theselecting of set points is described below with reference to FIG. 3.

FIG. 3 shows speed setting points. Along the horizontal axis the discradial position 40 is indicated. Along the vertical axis the rotationspeed 41 is indicated. A first curve for CLV 43 indicates the rotationspeed during the recording process. As a practical example the speed ofrecording is 2.4× the nominal speed of the DVD system. For reading afirst setting point 44 at a rotation speed of 25 Hz is indicated as ahorizontal line. A second set point 45 is positioned at 30 Hz, a thirdsetting point 46 at 40 Hz and a fifth setting point 47 at 50 Hz. In theexample in the fifth setting 47 the linear speed at reading variesbetween 2.1× and 5.5× the nominal reading speed for the DVD system. Thesetting of reading speed occurs as follows. According to the mode switchunit 31 the recording process may be interrupted at WRITE position 48.The rotation speed of the record carrier will be around 33 Hz. Forreading the closest setting point 45 is selected and the motor iscontrolled according to the set point of 30 Hz. Reading may startimmediately after the head reaches the new radial READ position 49. Thejump to this position will take some time, and the motor speed mayalready be adapted. In an embodiment the reading starts at the newradial READ position 49 before the speed is adapted completely. If anerror during reading occurs, the control unit 20 may perform a retry atthe same location as soon as the motor speed has been adjusted by againjumping to that position. The number of retries for reading an erroneouslocation may be increased, because the reading speed has not yet reachedits target value.

In an embodiment the speed selector 38 has a lowest speed setting for arotation speed substantially above the lowest rotation speed in thewrite mode. As shown in FIG. 3 the lowest speed of recording profile 43is 22 Hz. The lowest rotation rate setting point is 25 Hz, whichincreases the minimum speed at reading by circa 15% compared to asetting of 22 Hz. In an embodiment the speed selector 38 has a highestspeed setting at 50 Hz, while the maximum rotation speed duringrecording according to the CLV profile at 2.4× nominal DVD speed is 55Hz. The highest rotation speed during reading is 10% lower than themaximum rotation speed during recording. Hence the maximum linear speedfor reading is limited to around 5.5× DVD. Using the same speed as themaximum recording speed would have resulted in a maximum linear readingspeed of over 6×.

In an embodiment at least a number of the speed settings are at rotationspeeds having a predefined rotation frequency interval. The interval maybe equal for a number of settings. As shown in FIG. 3 there are settingpoints 45, 46, 47 at the interval of 10 Hz. Keeping the interval at thesame value results in a maximum speed difference of half the interval,e.g. 5 Hz. However the interval may also be chosen according to otherdesign criteria, e.g. the interval being smaller at higher rotationspeeds for creating a substantially equal response time for speeding upto each of the settings setting. In an embodiment the speed control unit37 is arranged as shown in FIG. 3 for accommodating a write rotationspeed range for recording in which range the highest speed issubstantially 2.5 times the lowest speed, and the speed selector isarranged for selecting one of 4 speed settings for the read mode.

In an embodiment the device has a write buffer 35 for storinginformation to be recorded, and wherein the mode control unit 31 isarranged for switching the modes in dependence on a filling degree ofthe write buffer 35. The mode control unit 31 is arranged for recordingthe input stream of real-time information via the write buffer and for,at the same time, retrieving of real-time information by alternating thewrite mode and the read mode. The writing continues until the writebuffer is substantially empty. The mode control unit 31 detects thefilling degree and switches to read mode. Usually also a read buffer 32is available. The read process continues until the write buffer issubstantially full, or alternatively until the read buffer issubstantially full. The buffer sizes are chosen so that switchingbetween data streams to be recorded and retrieved simultaneously doesnot occur too often, as this would cause wear and audible noise. Apractical value for the switch cycle is 3 to 10 seconds, for example 5seconds write, 0.5 second for a jump, 4 seconds read and again 0.5second for a jump. At a data rate of the data stream of about 5 Mbit/secthe write buffer 35 needs to be about 4 Mbyte.

FIG. 4 shows simultaneous recording and retrieving by alternatinglywriting and reading. The upper diagram 50 shows along a time axis thewriting of data by W_(n) wherein the index n indicates the order intime. On a first segment of the axis a writing action takes placeindicated by W. In the middle segment there are shown alternatingwriting and reading actions W₁ to W₅ and R₁ to R₅. The middle diagram 51schematically shows a first example of the writing and reading actionson two radial positions, reading position 53 being more inward andwriting position 54 being more outward. The lower diagram 52schematically shows a second example of the writing and reading actionson two radial positions, writing position 55 being more inward andreading position 56 being more outward.

FIG. 5 shows rotations speeds during alternatingly writing and reading.

Along the horizontal axis 61 the time indicated, along the vertical axis62 the rotation speed of the disc. The writing actions W₁ W₂ and W₃ aresuppose to take place in successive radial positions and therefore areshown to have the a decreasing writing rotation rate 63 according to theCLV profile. The first reading action R₁ takes place at a more inwardposition at a slightly higher first rotation speed selection 64, whilethe second and third reading actions R₂ and R₃ take place at a slightlylower second rotation speed selection 65. The rotation speed selections64, 65 are selected based on the actual rotation rate 63 during writing.Hence the difference between the rotation rate during writing and therotation rates during reading is limited. If always the closest readingspeed is selected, the maximum difference is half the interval betweenrotation rate settings 64, 65.

Although the invention has been mainly explained by embodiments usingoptical discs for storing video, the invention is also suitable forother disc type record carriers such as magnetic discs or any otherrendering device for any type of information that uses differing speedsfor reading and recording of information. It is noted, that in thisdocument the word ‘comprising’ does not exclude the presence of otherelements or steps than those listed and the word ‘a’ or ‘an’ precedingan element does not exclude the presence of a plurality of suchelements, that any reference signs do not limit the scope of the claims,that the invention may be implemented by means of both hardware andsoftware, and that several ‘means’ or ‘units’ may be represented by thesame item of hardware or software. Further, the scope of the inventionis not limited to the embodiments, and the invention lies in each andevery novel feature or combination of features described above.

1. Device for recording information on a disc-shaped record carrier, therecord carrier comprising a track for recording information, the devicecomprising a head for scanning the track, a read unit for retrievinginformation from the track via the head, a write unit for recordinginformation in the track via the head, a mode control unit for switchingthe device either to a read mode or to a write mode, and a rotationspeed control unit for setting the rotation speed of the record carrier,characterized in that the rotation speed control unit comprises a speedselector for selecting one of at least two speed settings for the readmode in dependence on an actual rotation speed of the record carrierduring the write mode when switching from write mode to read mode, thedifference in rotation speed between said actual rotation speed and thespeed in the read mode being limited by said selection.
 2. Device asclaimed in claim 1, wherein the speed control unit is arranged forcontrolling the speed of the record carrier during recording accordingto a constant linear velocity profile.
 3. Device as claimed in claim 1,wherein the speed control unit is arranged for controlling the speed ofthe record carrier during reading according to a constant angularvelocity profile.
 4. Device as claimed in claim 1, wherein the speedselector comprises a lowest speed setting for the read mode for arotation speed substantially above the lowest rotation speed in thewrite mode, and/or a highest speed setting for read for a rotation speedsubstantially below the highest rotation speed in the write mode. 5.Device as claimed in claim 3, wherein at least a number of the speedsettings are at predefined rotation frequencies having at least onepredefined rotation frequency interval.
 6. Device as claimed in claim 1,wherein the speed control unit is arranged for accommodating a writerotation speed range for recording in which range the highest speed issubstantially 2.5 times the lowest speed, and the speed selector isarranged for selecting one of 4 speed settings for the read mode. 7.Device as claimed in claim 1, wherein the device comprises a writebuffer for storing information to be recorded, and wherein the modecontrol unit is arranged for switching the modes in dependence on afilling degree of the write buffer.
 8. Device as claimed in claim 7,wherein the device comprises a video encoding unit for receiving videodata and providing encoded video as information to be recorded via thewrite buffer.
 9. Device as claimed in claim 7, wherein the mode controlunit is arranged for recording a first continuous stream of real-timeinformation via the write buffer and for, at the same time, retrieving asecond stream of real-time information by alternating the write mode andthe read mode.
 10. Method of controlling a speed of rotation of adisc-shaped record carrier, the record carrier comprising a track forrecording information, the method comprising scanning the track via ahead, retrieving information from the track via the head, recordinginformation in the track via the head, switching the device either to aread mode or to a write mode, and setting the rotation speed of therecord carrier, characterized in that the method comprises selecting oneof at least two speed settings for the read mode in dependence on anactual rotation speed of the record carrier during the write mode whenswitching from write mode to read mode, the difference in rotation speedbetween said actual rotation speed and the speed in the read mode beinglimited by said selection.